A method is known for preparing stretched porous polytetrafluoroethylene (ePTFE) by paste extruding or rolling a polytetrafluoroethylene (PTFE) fine powder and a lubricant, and then drawing the article after the lubricant has been removed.
The stretched product ordinarily has a microstructure of nodes interconnected with numerous generally parallel fibrils. Commonly, after the lubricant is removed, the PTFE article, usually in film form, is stretched in its unsintered state. The basic technology for this is found in U.S. Pat. No. 4,187,390 and U.S. Pat. No. 3,953,566.
A PTFE with a low amorphous content and a degree of crystallization of at least 98% is considered to be the PTFE fine powder that is best suited to this method. When such a PTFE fine powder is mixed with a mineral spirit, naphtha, or another such lubricant, it absorbs this lubricant and turns into a paste. It is well known that this PTFE paste can be economically molded by extrusion molding, calender molding, or another molding method that imparts shear deformation. The paste is usually molded into a tube, a rod, a tape, or some other such cross sectional shape. After molding, the lubricant is removed from the molded article, usually by drying. Then, after the lubricant has been removed, the molded article is drawn, i.e., stretched, to give it a porous structure. With the method that involves drawing in an unsintered state, the molded article from which the lubricant has been removed is drawn in the direction of one or more axes at a temperature below the melting point of PTFE, but preferably near the melting point. After drawing, the porous molded article usually is heated to a temperature above the melting point of PTFE and then cooled, in order to fix the microstructure of the article. The degree of sintering of the product is controlled by adjusting the maximum temperature or the length of time at which the article is kept at this temperature as dictated by the intended use of the finished product. Depending on the application, there are cases in which no sintering treatment is performed.
A molded article produced by drawing in an unsintered state becomes porous, and is filled with micropores. The nature of the microstructure is determined by the drawing temperature, drawing rate, draw ratio, and other factors. The microstructure is composed of a vast number of fibers and nodules that are linked together by the fibers, and the size and arrangement of the nodules will vary with the drawing conditions. For instance, if the article is drawn uniaxially, the nodules will be arranged in the form of islands at a right angle to the drawing direction, and the fibers that link the nodules together will be arranged parallel to the drawing direction. And when the article is drawn biaxially, the nodules will consist of particles or an aggregation of a few to hundreds of particles of fine powder, and the fibers that link them together will be oriented two-dimensionally from the nodules, with the degree of this orientation varying with the drawing conditions. With conventional porous films, the fiber diameter is very fine (approximately 0.1 .mu.m), and nodules are very large, in some cases reaching approximately 400 .mu.m.
A characteristic of this approach to making a porous article by drawing in an unsintered state is that the size in the direction at a right angle (perpendicular direction) to the molded article drawing direction does not vary in the drawing process. In other words, there is only minimal change in the thickness and width of the molded article with uniaxial drawing, and in the thickness of the molded article with biaxial drawing. This indicates that an increase in volume is the result of an increase in porosity, i.e., a decrease in density. This increase in porosity is caused by an increase in the voids, i.e., the space, between the nodules, and by the larger space that is created as the number and length of fine fibers increase. Consequently, with a method that involves drawing in an unsintered state it is fundamentally difficult to manufacture a film that is thinner than the original molded article.
Thin, porous membranes of stretched PTFE are taught in U.S. Pat. No. 4,902,423 to Bacino, but the membranes have very large pores.
Stretched porous PTFE films having small pore sizes (for microfiltration) and small nodes are described in U.S. Pat. No. 5,234,739 (Daikin, Ind.) to Tanaru, et al., but they are produced by stretching a semi-sintered PTFE.